docs: def helix(Di, rs, pitch, N, fd):
      r'''Returns loss coefficient for any size constant-pitch helix
      as shown in [1]_. Has applications in immersed coils in tanks.
  
      .. math::
          K = N \left[f\frac{\sqrt{(2\pi r)^2 + p^2}}{d} + 0.20 + 4.8 f\right]
  
      Parameters
      ----------
      Di : float
          Inside diameter of pipe, [m]
      rs : float
          Radius of spiral, [m]
      pitch : float
          Distance between two subsequent coil centers, [m]
      N : float
          Number of coils in the helix [-]
      fd : float
          Darcy friction factor [-]
  
  
      Returns
      -------
      K : float
          Loss coefficient [-]
  
      Notes
      -----
      Formulation based on peak secondary flow as in two 180 degree bends per
      coil. Flow separation ignored. No f, Re, geometry limitations.
      Source not compared against others.
  
      Examples
      --------
      >>> helix(Di=0.01, rs=0.1, pitch=.03, N=10, fd=.0185)
      14.525134924495514
  
      References
      ----------
      .. [1] Rennels, Donald C., and Hobart M. Hudson. Pipe Flow: A Practical
         and Comprehensive Guide. 1st edition. Hoboken, N.J: Wiley, 2012.
      '''
      return N*(fd*((2*pi*rs)**2 + pitch**2)**0.5/Di + 0.20 + 4.8*fd)